1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy. More particularly, this invention relates to a hydrogen-absorbing alloy having a body-centered cubic lattice structure which has a periodical structure generated by spinodal decomposition, has a large hydrogen storage capacity, has excellent hydrogen desorption characteristics and can mitigate an activation condition.
2. Description of the Prior Art
Solar energy, atomic power, wind power, geothermal heat, re-utilization of waste heat, etc, have been proposed as new energy sources to replace fossil fuel from the aspect of the environmental problems of the earth. In any of these cases, the common problem is how to store and transport energy. A system which electrolyzes water by using solar energy or water power and uses the resulting hydrogen as an energy medium can be said to provide ultimate clean energy in the sense that the starting material is water and the product obtained by consuming this energy is water, too.
As one of the means for storing and transporting this hydrogen, a hydrogen-absorbing alloy can absorb and store a hydrogen gas to a capacity about 1,000 times the volume of the alloy itself, and its volume density is substantially equal to, or greater than, that of liquid or solid hydrogen. It has long been known that metals and alloys having a body-centered cubic lattice structure (hereinafter called the "BCC structure"), such as V, Nb, Ta, Ti--V alloys, etc, absorb and store greater amounts of hydrogen than an AB.sub.5 type alloy such as LaNi.sub.5 and an AB.sub.2 type alloy such as TiMn.sub.2 that have been already put into practical application. This is because the number of hydrogen absorbing sites in the crystal lattice is large in the BCC structure, and the hydrogen absorbing capacity according to calculation is as great as H/M=2.0 (about 4.0 wt % in alloys of Ti or V having an atomic weight of about 50).
A pure vanadium alloy absorbs and stores about 4.0 wt %, which is substantially similar to the value calculated from the crystal structure, and desorbs about half this amount at normal pressure and room temperature. It is known that Nb and Ta as the elements of the same Group 5A of the Periodic Table exhibit a large hydrogen storage capacity and excellent hydrogen desorption characteristics in the same way as vanadium.
Because pure V, Nb, Ta, etc, are extremely high in cost, however, the use of these elements is not realistic in industrial applications which require a considerable amount of the alloys such as a hydrogen tank or a Ni--MH cell. Therefore, properties of alloys have been examined within the range having a BCC structure such as Ti--V, but new problems have arisen in that these BCC alloys merely absorb and store hydrogen at a practical temperature and pressure but that their hydrogen desorption amount is small, in addition to the problems encountered in V, Nb and Ta in that the reaction rate is low and activating is difficult. As a result, alloys having a BCC phase as the principal constituent phase have not yet been put into practical application.
The conventional attempt to control the characteristics by alloying has been carried out by component design in all of the AB.sub.5 type, the AB.sub.2 type and the BCC type. However, the set range of the component does not exceed the category of the intermetallic compound single-phase and the BCC solid solution single-phase in all of these examples. Japanese Unexamined Patent Publication (Kokai) No. 59-78908 is an example of the prior art references in this field. As a method of producing a body-centered cubic lattice type alloy composition and its hydrides at room temperature, this reference discloses a method of producing metal hydrides which comprises reacting (a) a body-centered cubic system structure containing titanium and a second metal selected from the group consisting of molybdenum, vanadium and niobium, (b) a solid solution alloy containing at least about 1 atm % of a third metal selected from the group consisting of aluminum, cobalt, chromium, copper, manganese, nickel, iron, gallium, germanium, silicon and mixtures thereof under the state where the third metal is dissolved in the body-centered cubic structure system, when the second metal is vanadium or niobium or, whenever desired, when the second metal is molybdenum, and a hydrogen gas at a temperature of from about 0 to about 100.degree. C., whereby the reaction rate between the solid solution and hydrogen at this temperature is at least about 10 times the reaction rate between non-alloy titanium and hydrogen at this temperature and at an equal hydrogen pressure.
However, this prior art reference does not describe the case other than the solid solution single-phase at all, although the two-phase region exists in Ti--V and Ti--V--Fe systems in the case of the alloys having the BCC structure. Further, although the technology of this reference can mitigate the reaction rate and the activation condition, it cannot improve the desorption characteristics per se, that is, the mitigation of the desorption temperature and the pressure condition.
Several attempts have been made recently to obtain multi-phase alloys. For example, Japanese Examined Patent Publication (Kokoku) No. 4-80512 (corresponding to U.S. Pat. No. 4,623,597) discloses an extremely broad concept including the single-phase and the multi-phase without specifying the crystal structures of the alloy phase. Though the patents and the researches of the hydrogen-absorbing alloys have been limited in the past to the category of the single-phase intermetallic compounds, this prior art reference describes the technology for controlling the optimum structures such as the combination of the multi-phases and the structures that can fully exploit the effects as the hydrogen-absorbing alloys, though the reference does not define the combinations of the phases, the structures and the components that give concrete effects. The Examples of the reference disclose multi-phase alloys having a crystallographically random structure originating from an amorphous phase, for quenched films.
Further, other prior art references include research papers ("Science", Vol. 260 (1993), pp. 176; "Electrical Steel Making", Vol. 66 (1995), pp. 123), and so forth. These references describe the deviation of the components from the stoichiometric composition of a Laves phase as an intermetallic compound in the AB.sub.2 alloy and the second phase that appears due to the addition of the third and fourth elements. However, these papers describe that the Laves phase as the principal phase exhibits the fundamental effects as the hydrogen-absorbing alloy, that is, the hydrogen-absorbing capacity, the hydrogen desorption temperature, the equilibrium pressure, etc, while the second phase is small in amount and is limited to the accompanying effects such as a mitigation of the activation condition, an improvement in durability, and so forth. As described above, the multi-phase technology according to the prior art has not yet succeeded in accomplishing a drastic increase of the hydrogen absorbing capacity and the mitigation of the absorption and desorption condition. The development of the technology capable of further improving these characteristics has therefore been desired.